1. Field of the Invention
The present invention relates to the field of silica crucibles and more particularly to a silica crucible having a multi-layer wall with a barium-doped inner layer.
2. Background of the Invention
The Czochralski (CZ) process is well-known in the art for production of ingots of single crystalline silicon, from which silicon wafers are made for use in the semiconductor industry.
In a CZ process, metallic silicon is charged in a silica glass crucible housed within a susceptor. The charge is then heated by a heater surrounding the susceptor to melt the charged silicon. A single silicon crystal is pulled from the silicon melt at or near the melting temperature of silicon.
Some CZ processes, e.g., those that are heavily doped and those used to make ingots for solar cells, have very high temperatures or very high reactivity between the melt and the crucible. When pulling an ingot from which solar cell wafers are cut very high efficiency is required. This requires intense heat for fast melting at the initial stages as well as a prolonged run. These types of CZ processes emphasize the need to improve the useful life of the crucible.
At operating temperatures, the inner surface of a silica crucible frequently reacts with the silicon melt. In many cases, the inner surface of the crucible undergoes a change in morphology. The inner surface of a crucible is seen to roughen during prolonged operation in a CZ run.
This roughening can cause a loss of crystal structure of the pulled ingot. Inner surface roughening renders the crucible unfit for use in silicon ingot manufacture. When a major portion of the inner surface of the crucible is covered by a rough surface, crystalline structure is disrupted at the crystal-melt interface. Such a roughened crucible is unsuitable for ingot manufacture and silicon crystal pulling using a roughened crucible must be ceased to avoid manufacture of substandard ingots.
Additionally, the inner surface of a silica glass crucible can partially dissolve into the silicon melt during the CZ process. Silicon and oxygen, the main components of a silica crucible, are not deleterious to the silicon melt. However, impurities in the inner layer of the crucible can be transferred to the silicon melt during this process. The quality of the pulled single crystal may be ruined, depending on the extent of contamination and the nature of the contaminant.
One such effort to control inner surface morphology is a crucible with barium-containing chemicals coated onto the inner surface, which promotes devitrification on the inner surface of the crucible. This is the phase transformation from amorphous silica to crystalline silica. Devitrification prevents particulate generation at the silica-melt interface. The devitrified layer, created during a CZ run, comprises a crystallized silica layer and is reported to dissolve uniformly and maintain a smooth crucible inner surface.
If the crystalline layer so formed is too thick, the volume change caused by the phase transformation results in cracking of this layer. This permits melt penetration between the crystallized and amorphous layers, which can ultimately result in the cracked layer pealing off.
In addition, bubble expansion in the undoped crucible wall beneath the doped layer releases gas that may result in pitting of the doped layer on its inner surface, even when the inner surface is not subject to cracking as a result of expansion during devitrification.